Context. Developing effective selection strategies to enhance efficiency and productivity remains a priority for Merino sheep producers in Australia. Aims. Beyond traditional approaches that rely on liveweight and energy-based assumptions, this study aimed to provide more precise insights into sheep efficiency and profitability by examining genetic parameters for 15 body composition traits. The study estimated the heritability of traits, including liveweight and body condition score, wool growth and leptin concentration, ultrasound measurements of fat and muscle depth, direct whole-body composition measured using dual-energy X-ray absorptiometry (DEXA) and predicted DEXA from ultrasound and liveweight. Methods. Multiple records were collected on 640 wethers aged almost 30 months and 1023 ewes aged 5.5 years, born in 2016 and 2017 and bred from 29 different sires in the Merino Lifetime Productivity (MLP) project. Key results. Direct heritability estimates from a univariate repeated animal measurement model, ranged from low for wool growth and leptin concentration (0.14 ± 0.06 to 0.22 ± 0.06), moderate for liveweight, body condition score and ultrasound fat and muscle depth (0.37 ± 0.08 to 0.48 ± 0.04), moderate to high for DEXA whole-body composition (0.42 ± 0.11 to 0.66 ± 0.14), moderate for predicted whole-body composition (0.17 ± 0.05 to 0.21 ± 0.06). Repeatability estimates were low for wool growth and leptin concentration (0.22 ± 0.06 and 0.23 ± 0.06), moderate for condition score (0.48 ± 0.04) and predicted DEXA measurements (0.58 ± 0.02 to 0.64 ± 0.02), and moderate to high for liveweight (0.71 ± 0.01), ultrasound measurements (0.49 ± 0.02 to 0.61 ± 0.01), and DEXA measurements (0.52 ± 0.03 to 0.95 ± 0.01). Maternal permanent environmental effects were insignificant for most measurements. Conclusion. The heritability and repeatability of DEXA whole-body composition traits were higher than those of body condition score and ultrasound-measured fat and eye muscle depth. Implications. These findings provide information on the genetic variation and repeatability of wool growth and body composition traits that can inform future selection strategies in Merino sheep. The relatively high heritability and repeatability of DEXA-derived traits indicate their potential as a reference technology for body composition assessment in Merino sheep. Further evaluation of genetic relationships with production, reproduction, health traits, simpler indicator traits and their economic value is required to determine their suitability for Merino breeding programs.